In the prior art, outer plates of domestic electrical appliances were generally produced by forming, processing and setting up cold rolled steel sheets or zinc type plated steel sheets, followed by performing phosphate treatment and finally melamine, acrylic polyester type coating. Aiming at rust-proof properties, chromate treatment was widely made on the steel sheets, instead of phosphate treatment.
Since satisfactory corrosion resistance and paint adhesion could not be secured only with the phosphate and chromate treatments, steel sheet plated with double structured layers of an organic resin film (organic composite silicate film) on the chromate treatment was recently proposed in Japanese Patent Publication No.60-33192, and it has been reduced to practice as a coated substrate steel sheet, omitting the phosphate treatment.
Of this multi-layered steel sheet, the organic resin film of the upper layer is composed of a water dispersible silica sol and water soluble or water dispersible organic resin and silane compound.
However, for improving the corrosion resistance, the chromate film and the organic resin film of the multi-layers should be thick. Due to the thickness of the film, problems occur such as peeling of the film, deterioration of the paint adhesion and lowering of weldability under severe conditions. It is difficult to satisfy the requirements of (a) paint adhesion of high degree, (b) high corrosion resistance and weldability concurrently.
On the other hand, in the domestic electrical appliances, so-called fabricator precoated steel sheet, where the steel sheet is painted in advance and deformed by a roll-forming, has gradually become the trend of the times because of rationalization of the manufacturing process. A big problem with fabricator precoated steel sheet is to secure very severe paint adhesion durable to the roll forming and enough corrosion resistance.
For fabricator precoated steel sheet, not only the foregoing phosphate or chromate treatments but also the multi-layered steel sheet of Japanese Patent Publication No. 60-33192 lack sufficient paint adhesion and corrosion resistance, and there have been demands for paint adhesion excellent in forming process and corrosion resistance excellent after the forming process.
The present invention has been realized in view of problems involved about the prior art for providing multi-layered steel sheets having excellent paint adhesion, corrosion resistance and weldability, especially as plated substrate steel sheets for the fabricator precoated steel sheet.